Exploration of pH-Driven Morphology and Defect Evolution in Nickel Cobalt Carbonate Hydroxide Nanomaterial for Supercapacitor Applications.

Nickel cobalt carbonate hydroxide (NCCH) nanostructures with tunable morphologies, crystallite sizes, and defect structures are synthesized using a pH-modulated hydrothermal approach to explore the correlation between structural properties and electrochemical performance. Significant variations in crystallinity, surface area, chemical structure, and morphology are observed, as confirmed by synchrotron X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller analyses. Among the samples, the one synthesized at pH = 8.25 exhibits the most optimized physicochemical characteristics, including the highest surface area, smallest crystallite size, and a unique dual-phase nanowire-on-nanosheet morphology. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy analyses reveal the presence of abundant transition metal vacancies and/or oxygen interstitials on the surface. These defect-engineered features result in exceptional electrochemical performance, delivering a high specific capacitance of 3061.1 F g at 0.5 A g, 2620.0 F g at 1 A g, and 1533.3 F g at 10 A g, and ≈99% capacitance retention over 5000 cycles at current density of 5 A g. This study underscores the effectiveness of pH modulation in designing defect-rich NCCH nanostructures for high-performance supercapacitor applications, establishing a clear link between structure, defects, and electrochemical behavior.